1. Field of the Invention
This invention relates to a method and apparatus for controlling an atmosphere in a heat treatment furnace, and more particularly relates to a method of and apparatus for controlling an atmosphere in a heat treatment furnace for carrying out a gas carburizing, carbonitriding or bright controlled atmosphere heat treatment, etc.
2. Description of the Prior Art
In conventional heat treatment methods, such as gas carburizing of metals, a mixture of a hydrocarbon gas and air is converted into an endothermic gas through the use of an endothermic type converted gas generator. The endothermic gas is introduced into the furnace, and an enriched hydrocarbon gas is added to the furnace in order to obtain a predetermined carbon potential.
However, recently, in order to enhance the quality, and to reduce the treatment time and running cost, the gas generators are no longer desirable. Rather, introducting a hydrocarbon gas and an oxidizing gas are directly into the furnace to carry out the carburizing in the furnace has been proposed.
Such method is described in Japanese Patent Applications Laid-Open Nos.159567/1986 and 63260/1992.
However, in the method shown in the Japanese Patent Application Laid-Open No.159567/1986, the oxidization gas to be added in the furnace is oxygen, the partial pressure of CO is approximately 29% when CH.sub.4 is used as the hydrocarbon gas, and the partial pressure of CO is approximately 38% when C.sub.4 H.sub.10 is used as the hydrocarbon gas. In the method shown in the Japanese Patent Application Laid-Open No.63260/1992, the partial pressure of CO is approximately 40% in case that CO.sub.2 is used and butane is used as the hydrocarbon gas. According to the conventional methods, the carburizing time can be shortened, because the partial pressure of CO is higher than that in the other normal method. However, the oxidization at the grain boundary layer of the goods to be treated is promoted.
Furthermore, the partial pressure of CO in the atmosphere in the furnace fluctuates because a large quantity of air is introduced into the furnace when the goods to be treated are inserted into and taken out of the furnace. In the method shown in the Japanese Patent Application Laid-Open No. 63260/1992, the quantity of the hydrocarbon gas to be supplied into the furnace is controlled so that the carbon potential in the atmosphere becomes constant. However, in fact, the atmosphere is varied to a large extent according to the change of the type (weight and surface area) of goods to be treated, and accordingly the fluctuation of the carbon potential becomes large, so that the fluctuation in the surface carbon contents of steel becomes large.
Furthermore, the carburizing speed in the direct carburizing method is varied on a large scale according to the carburizing time and the diffusion time. In the carburizing time, the main effect is the direct decomposition of the hydrocarbon gas, etc. (raw gas) and in the diffusion time, the main effect is the Boundouard reaction.
In the carburizing time, the degree of the decomposition varies due to the quantity of the hydrocarbon gas to be introduced directly into the furnace and the temperature of the atmosphere in the furnace as well as the type of goods to be treated in the furnace. As a result, hydrocarbon gas in excess of the amount required for carburizing is piled as a soot in the furnace, thereby potentially subjecting the goods to be treated to soot.
If the heat treatment is carried out in the sooting range, the service life of the oxygen sensor becomes reduced.